Chronic myelogenous leukemia (CML) is characterized by increased formation of granulocytes and other blood forming elements. Despite the name, CML is almost invariably fatal, with a median survival time of approximately three years. Conventional chemotherapy for the initial chronic phase has failed to prevent transformation to acute leukemia or to significantly affect survival. Although bone marrow transplantation is successful in a minority of patients, clearly more effective regimens, to be administered during the chronic phase, need to be developed. We previously demonstrated that gamma interferon (IFN-gamma) has a potent suppressive effect on hematopoiesis in vitro and have provided evidence implicating IFN-gamma in the pathogenesis of the hematopoietic suppression observed in aplastic anemia. Another of the interferons, alpha interferon (IFN-alpha), has shown promising clinical results in the treatment of chronic phase CML. Based on theoretic advantages and in vivo suppression of myelopeiesis observed in patients treated with recombinant IFN-gamma (rIFN-gamma) for other disorders, we embarked on a clinical study of patients with both chronic and accelerated phases of CML with rIFN-gamma approximately one year ago.